DESCRIPTION (provided by applicant: Identifying anti-apoptotic signaling pathways driving therapy resistance and relapse in breast cancer (BC) is essential for developing effective strategies for improving outcome in patients with aggressive breast cancer (BC). To this end, we have identified an important role for glucocorticoid receptor (GR)-mediated anti- apoptotic signaling in both in vitro and in vivo pre-clinical models of GR-expressing human breast cancer (BC). Specifically, in the previous funding period we identified mechanisms underlying direct cross-talk between GR signaling and the PI3-K and MAPK pathways through identifying GR-regulated genes encoding kinases (e.g. SGK1) and phosphatases (e.g. MKP1) required for GR-mediated cell survival in estrogen receptor, progesterone receptor and HER2-negative BC (triple-negative BC or TNBC) models. This renewal proposes to expand our studies to achieve a comprehensive understanding of pathways mediated by glucocorticoid receptor (GR)-mediated cell survival in TNBC. We have collected, curated, and analyzed data from several early BC gene expression studies with long-term clinical follow-up and examined the association between high GR expression in primary BCs and risk of relapse in over 1300 patients. We were excited to find that high GR expression in estrogen receptor-alpha (ER)-negative BCs indeed associates with a significantly increased risk of early relapse, supporting our previous discoveries. More interestingly, we unexpectedly found that risk of relapse was reduced in patients with ER-positive, GR over expressing tumors. This exciting finding opens up a new area of study: Investigating how GR-signaling differs in BC depending upon ER context. We hypothesize that GR-mediated gene expression in ER-negative BC (including TNBC) activates genes and pathways that are specifically antagonized by ER activity in ER+ tumors. We now propose to 1) use ER antagonism of GR transactivation as a tool to identify additional critical GR target genes and pathways underlying therapy- resistant TNBC, 2) to identify mechanisms underlying ER antagonism of GR transactivation, and 3) to test the function of ER-antagonized GR target genes and pathways in preclinical TNBC models. The results of these experiments are expected to expand options for high-risk ER-negative and TNBC patients by identifying previously unknown GR-mediated cell survival pathways contributing to therapy resistance and early relapse. PUBLIC HEALTH RELEVANCE: Our laboratory has discovered that glucocorticoid receptor (GR) expression identifies a very poor prognosis group of patients with early-stage estrogen receptor-negative (ER-) and triple negative (ER-/PR- /HER2-) breast cancer (TNBC). Surprisingly, GR expression in ER-positive tumors is associated with a relatively good clinical outcome. Our previous pre-clinical work demonstrated that GR activation of specific target genes protects BC cells against chemotherapy-induced apoptosis. We hypothesize that ER activity antagonizes GR transcriptional function in a gene-selective manner, thereby reducing the expression of a subset of GR target genes likely to mediate chemotherapy resistance and relapse in ER- breast cancer. We propose to use ER antagonism of GR activity as a tool for identifying functionally relevant genes and pathways underlying poor prognosis TNBC. These downstream GR pathways can then be tested as novel therapeutic targets in TNBC.